Recently, the automobiles or various electric devices are required to save energy and the motors provided on them need to possess a high efficiency and also need to be downsized. Thus, it will be necessary to develop magnets which are smaller in size and have higher magnetic forces.
It is known that the rare earth based magnet composed of rare earth element-containing R-T-B based alloys has the most powerful magnetic force presently. The rare earth based bonded magnet obtained by mixing the alloy powders and the resins and then subjecting the mixture to the molding process can be formed into any shape. Thus, this kind of magnet is easier to be molded as sheets and then used in small sized motors. Up to now, isotropic alloy powders are mainly used. However, the rare earth based bonded magnet with a higher magnetic force is under development which is obtained by using anisotropic alloy powders.
However, the rare earth based bonded magnets are hardly used in motors which work under hot environments, such as the motor used in the engine compartment of the automobiles. One of the reasons is that the coercivity of the alloy powders used in the rare earth based bonded magnets is not high enough that serious demagnetization happens at a high temperature. If the rare earth based bonded magnet can be used in a hot environment within a car, it promises to play a big role in the energy saving.
The method for preparing the R-T-B based anisotropic alloy powders for the rare earth based bonded magnets includes the HDDR method in which the hydrogenation process, decomposition process, desorption process and recombination process are performed in order. With the HDDR method, the orientation of the original crystallographic axis in the alloy raw materials can be maintained while the crystal grains can be micronized to a size of several hundreds of nanometers. In this way, anisotropic alloy powders can be obtained with high coercivity.
During the preparation of the alloy powders with the HDDR method, the preparation conditions can be adjusted to improve the magnetic properties of the alloy powders. For example, it is suggested in Patent Document 1 that the reaction rate can be changed by controlling the atmosphere during the desorption process and recombination process so as to improve the magnetic properties of the alloy powders. That is, the reaction rate in the recombination process can be adjusted by controlling the releasing rate of hydrogen during the desorption process so that alloy powders can be obtained with a high coercivity.